1) Field of the Invention
This invention relates to magnetic circuits which use permanent magnets to create a magnetic field, specifically the invention relates to a magnetic circuit design used in magnetic particle clutches and brakes which are used to create a resisting force for linear or rotary motion.
2) Background of the Prior Art
Magnetic particle devices have been used where long life and stable, consistent torque have been important. One example is tensioning of wire when winding electric coils. In this case, long brake life is important since the brake is slipping continuously as it applies tension to the moving wire. Furthermore, the tension must be stable and repeatable to ensure the coil is wound with consistent tension but not with tension so high that the wire stretches or breaks.
While numerous configurations have been created, many have involved complex designs which do not lend themselves to economical manufacturing and assembly procedures. For example, U.S. Pat. No. 3,374,375 requires horseshoe shaped magnets ground with a curvature to match the whell to which it is mounted. This requires expensive machining. The many internal parts increase assembly time. Also the more parts that are used, the chances for assembly errors such as misaligning components are increased. U.S. Pat. No. 4,681,197 does increase torque output per size but requires a more complex rotor design. This increases machining complexity and difficulty in assembly.
Another design, U.S. Pat. No. 3,497,160 uses an alternating magnetic field to produce torque. With this design, the torque producing element is on the inside of the electric coils. Since this necessitates a relatively small cross sectional area, torque is limited. Also heat is produced in this small, internal part which produces torque. Furthermore it is surrounded by electric coils which also produce heat. So even if the torque level can be increased, heat dissipation is limited thereby limiting the force and duty cycle of the device.
Other configurations are not intended to increase torque output per size. These designs do not address a major shortcoming of magnetic particle brakes which is their cost. More specifically, the ratio of torque produced to cost is much higher than devices which rely on friction surfaces rubbing together to produce torque. The high cost of magnetic particle brakes currently being produced limits the market potential of the product to low volume, specialty applications. For example, U.S. Pat. Nos. 2,741,714; 4,239,092; and 4,856,631 focus on features such as adjustable torque without considering how to increase the maximum torque output of the device.
U.S. Pat. No. 3,374,375 uses bucking fields in an eddy-current coupling. However it does not use them to increase magnetic field strength to produce more torque. Instead like poles facing each other are used to cancel out the magnetic field between them and produce less torque.
U.S. Pat. No. 5,337,862 does improve the efficiency of the magnetic circuit. The design provides incremental improvements to the efficiency of the magnetic circuit design butdoes not produce quantum improvements in torque. Also, the numerous magnets need to be accurately mounted on the support rings, this increases assembly costs and the chance for error in aligning the magnets.
Finally U.S. Pat. Nos. 4,844,220 and 4,974,706 are simple designs with low cost machining and assembly operations. The ability to increase torque output is limited due to the single permanent magnet used. Also the torque producing powder is placed on the face of the rotor instead of the periphery. This reduces the radius at which the torque producing powder is placed. Since the torque is calculated as force multiplied by the distance from the center of rotation, it follows that reducing the radius reduces torque output.
It is among the objects and advantages of the present invention to create a versatile magnetic circuit design which can be used in magnetic particle devices. Another object is to provide a magnetic circuit design with alternating magnetic polarity designed to increase the intensity of the magnetic field and thereby torque which can be produced for a given size device. Yet another object of the invention is to maximize torque and heat dissipation by placing the torque producing element as close to the outside diameter of the device as possible.
Another object of this invention is to provide a magnetic circuit design which uses low cost parts and simple assembly procedures thereby making high volume production economically possible.
A further object of the present invention is to provide a magnetic circuit design which can easily increase magnetic field strength with a relatively small increase in length of the clutch.
Still a further object of the present invention is to provide a magnetic circuit design which does not require the use of an external power supply to generate a magnetic field. This results in a lower total cost, self contained, reliable device. Further objects and advantages of the magnetic circuit design will become apparent from the consideration of the ensuing description and drawings.
The present invention is used as a magnetic particle clutch. The device comprises a rotary shaft assembly consisting of a series of magnetically conductive, round washers and permanent magnets stacked, with alternating magnetic polarity, along the length of the shaft. They are bonded to the shaft and concentric with the shaft. The magnets are mounted on each side of the washers with similar magnetic poles facing each other. This creates a very strong bucking magnetic field which passes radially outward through the washer.
A second assembly consists of a magnetically conductive cylinder with housings pressed into each end. The shaft assembly is mounted concentrically inside the cylinder assembly. The shaft assembly is relatively rotatable with respect to the cylinder by pressed in bearings which are mounted in each end housing.
The inside diameter of the cylinder is slightly larger than the outside diameter of the washers and magnets. An annular air gap is thereby created and filled with a fine, magnetic powder. The magnetic field created by the permanent magnets acts upon the magnetic powder and links the shaft assembly to the cover assembly.
There are numerous advantages to the configurations previously described. It has been shown experimentally that a series of thin permanent magnets mounted with alternating magnetic polarity produces more torque for a given axial length than one continuous magnet. This is due to the bucking magnetic fields and efficiency of the short magnetic flux path. Another advantage of the short flux path is that a thinner cylinder wall can be used and still contain the magnetic field. This minimizes the outside diameter of the clutch and rotational inertia of the cylinder assembly. Yet another advantage is the ability to increase torque of the clutch simply by stacking additional magnets and washers along the shaft. Still another advantage is the lower cost due to the economies of scale of using multiple identical parts.
Yet another advantage to the present invention is the torque producing element is as close to the outside diameter of the clutch as possible. Torque is defined as force multiplied by the distance from the center of rotation. By placing the torque producing element as far from the center of rotation as possible, torque is maximized. Also the cylinder is the primary means for removing heat. Heat is dissipated by convection and radiation to the surroundings. This reduces heat being conducted through the shaft to other components attached to the shaft. Heat dissipation is maximized since the heat, and torque, producing magnetic particle element is in direct contact with the cylinder.
The air gap is critical in producing the highest torque and in making torque consistent between clutches. With this invention, the gap is a function of the washer and cylinder diameters. This eliminates the chance for errors during assembly unlike axial gap designs which require careful assembly to compensate for stack-up tolerances of multiple axially mounted parts. The washers and magnets are located in position by sliding them over the shaft thereby further simplifying and increasing the accuracy of the clutch assembly.